The present invention relates to a seismic acquisition device for charting marine floors by seismic reflection, with a very high resolution and up to floor depths of as much as a hundred meters.
At present, in order to chart marine floors, use is made of a diverse range of seismic sources which generate acoustic waves. Each source is characterized by the frequency band or spectrum of the acoustic emission. With each spectrum there corresponds a depth of penetration of the wave into the floor and a specific resolution.
Two types of sources are commonly used: on the one hand, sediment penetrators or pingers which are sources operating with a transducer and on the other hand, boomers which generate acoustic waves using Foucault currents in two parallel metal plates. An exemplary sediment penetrator is described in patent DE 1 046 903. This source emits a spectrum which can be centered, on request, between 2 and 5 kHz and which has low penetration (between 0 and 10 m) but high resolution. For its part, the Foucault current acoustic wave generator emits a spectrum which is centered on 700 Hz and which therefore has greater penetration (between 0 and 70 m), but lower resolution in the region lying between 0 and 10 m.
This is why, at present, to obtain a good knowledge of marine sediments, it is necessary to employ both these types of sources simultaneously, so as to harness their respective qualities. However, this technique is penalizing from the operational standpoint on account of the fact that it is necessary either to tow both sources simultaneously and hence to record both sources at the same time on two different recorders, thereby causing annoying interference from an electronic standpoint, or to perform two successive passes, each of the passes being carried out with just one of the two sources. Moreover, this technique is scarcely satisfactory when operating at water depths of greater than 200 m.
The article by GETTRUST J. F. et al. xe2x80x9cDevelopment of a low-frequency, deep-towed geoacoustic systemxe2x80x9d of Sep. 24, 1990, pages 38 to 40, describes a seismic acquisition device known as a DTAGS. This device comprises a resonator formed of five piezoelectric rings disposed around a cavity and which emits over a frequency band lying between 250 and 650 Hz.
However, the performance of such a device is rather limited on account of the narrowness of its frequency band. Moreover, its considerable weight, 800 kg, renders it difficult to tow. Finally, the so-called xe2x80x9cdeep-towxe2x80x9d technique, which consists in towing the transducer at great depth, above the sea bed with a multitrace acquisition system is very difficult to implement.
U.S. Pat. No. 5,363,345, which represents the closest state of the art, discloses a so-called Janus-Helmholtz transducer which can be used in a marine environment for exclusively military applications. It serves mainly to detect submerged objects, such as submarines. This transducer comprises two electro-acoustic motors disposed on either side of a central mass and whose axes are aligned along a horizontal direction, two horns being fixed to the opposite ends of the motors.
Patent FR 1 397 142 relates to a seismic source which generates vibrations electrodynamically or hydraulically, these vibrations actuating two pulsating plates. However, with this source it is not possible to chart marine floors with good resolution.
Patent EP 0 209 238 relates to a seismic source of a different kind from that of the patent application, since it comprises an electrical pulse generator and a transformer for matching.
The present invention aims to remedy these drawbacks and proposes a seismic acquisition device using a single seismic source capable of replacing the sediment penetrators and boomers, that is to say a seismic source having much greater penetration than that of sediment penetrators, allied with high resolution over the entire range of depths within which it operates.
The invention also relates to a seismic acquisition device which can be employed at great depth in sea water without its performance being impaired. It also relates to a seismic acquisition device which is, in addition, of low bulk and modest weight, is very simple to employ and particularly economical.
To obtain these results, the invention proposes a device for seismic acquisition in a marine environment according to claim 1.
Thus, for the first time in marine seismic surveying, the invention proposes that a low frequency transducer with a wide frequency band and great depth of immersion be combined with a computer, a power amplifier and an impedance matcher, with a view to obtaining a high emission level over a very wide frequency band which is programmable at will.
The use of such a transducer for marine seismic surveying is in itself an inventive idea since, as was explained earlier, this transducer was used only to detect submerged bodies, such as submarines and for this particular application, the axis of the electro-acoustic motors and of the horns was oriented horizontally. In the application to marine seismic surveying which forms the subject of the present invention, it was necessary to envisage giving the said axis other orientations, for example the vertical direction, so as to direct the power of the acoustic waves optimally. Surprisingly, hitherto nobody has thought of using this transducer for marine seismic surveying, although it makes it possible to obtain considerable advantages. Specifically, acoustic wave emissions lying between a few hundred Hz to 10,000 Hz approximately are obtained at levels of around 200 dB over a large part of this frequency band.
The acquisition device according to the invention therefore has superior performance to that of the sediment penetrator and the boomer. Moreover, it is simpler, lighter and less bulky than these two sources.